https://bugzilla.kernel.org/show_bug.cgi?id=198123
--- Comment #26 from Daniel Vetter (daniel@ffwll.ch) --- Re #commment 24: crtc_commit is for modesets, the legacy helpers do _not_ call the DPMS functions in that case. radeon does what every reasonable legacy kms driver did and calls the dpms function from the prepare/commit hooks, but ast didn't do that. Hence why my patch fixed ast (but a similar patch for radeon doesn't make sense).